For the non-paraxial case, the phase mask at the input plane has to include features that are smaller than the wavelength and the relevant propagation distance is limited up to several wavelengths [1]}. In this regime, the Fresnel diffraction integral is invalid and the more general Rayleigh–Sommerfeld diffraction integral is used in order to properly propagate the input field [2]}:
\(u\left(x, y , z\right)=\frac{1}{i \lambda } \int _{-\infty }^{+\infty } \int _{-\infty }^{+\infty } u_0\left(\xi , \eta \right) \frac{z e^{i k R}}{R} \mathrm {~d} \xi \mathrm {d} \eta \) 
